Fix T69791: Fix crash reading old file browser in temporary window

When reading a old .blend file (from before the new file browser
design), we wouldn't create the execute region for existing file
editors. This usually wasn't an issue, but it could become one when a
file browser was opened in a temporary screen before, and that screen
was still visible. Then code spawning the new file browser would re-use
the old file browser data, assuming the execute region was there.

Handle this in versioning code and let rest of the code keep sane
assumtions (e.g. that there always is a execute region, even if
invisible).

UI: Refactor and improve buttton extra icons

Without this patch there could only be one superimposed icon
and the operators were hard coded for the button types.
This keeps the previous, sort of predefined extra icons working in a
rather generic way, but allows adding specific ones for specific case
through `UI_but_extra_operator_icon_set()`.

Reviewed by: Campbell Barton

Differential Revision: https://developer.blender.org/D5730

UI: Add superimposed + and - icons for file number

Brings back + and - icons to the file name button, but now as
superimposed icons.

Fix crash closing stacked file browser window

Caused by ab823176d31dc1.
Steps to reproduce were:
* Open Preferences
* Open file browser through Lights -> Install (doesn't matter which)
* Close browser through the window controlls

The window was freed earlier, but still referenced by new handler
context storage.

Fix T67756: File drag starts on file browser open

When the file browser was opened (from a temporary window since the file
browser redesign) using a button in the UI, under certain conditions
moving the mouse would trigger files to be dragged.

Note that this has been an issue before the new file browser design was
introduced, although under slightly different conditions.

Steps to reproduce:
* With factory settings, press F12
* Open a different image in the appearing Image Editor (not Render
Result)
* Make sure the window is not maximized
* Press N to open the side bar, open Image tab
* Click the folder icon there to change the image
* Change to thumbnail display type in the appearing file browser
* Cancel, click the folder icon again
Moving the mouse now would start dragging files in most cases.

The same issue could be reproduced in a similar way when installing
lights/MatCaps or HDRIs through Preferences -> Lights -> Install...

UI: Allow file number shortcuts ouside main region

Makes the numpad + and - type of shortcuts to increase/decrease the file
number suffix work in the upper and lower bar of the file browser.

Had to add keymap handlers to the execute region for this to work.

UI: Allow context menu in all file browser regions

Fix: File browser errors not reported in the UI

This is a partial fix, in that it only brings back the banner reports in
the status bar. The popups still don't show up but I need to investigate
more.
It's really ugly that reports rely on wmWindowManager.winactive, but
that's how it is...

Partialy fixes T69581.

Fix: File browser report popups not showing up

Popup would be closed immediately after creating, because of incorrect
mouse coordinates stored in window (popup is set to close if mouse is
some distance away from it).

Completes fix for T69581.

Fix saving images from temp Image Editor failing

Steps to reproduce were:
* Ensure //Render//->//Display Mode// is //New Window//
* F12
* In the opened Image Editor, Alt+S to save the image
* Save the image
The saving would fail silently.

Issue was that wm_handler_op_context() would fail to find the correct
area to activate, as the wrong window was active in context. So allow
overriding this window and do so when creating the file-select handler.