[ui] Improve "Clear Images" action's behaviour and performance

[cbentejac]

Description

This PR modifies the behaviour of the "Clear Images" menu action as follows:

• It is now executed on the Python side instead of the QML side, which makes it more than 4x faster:
  • For 500 PNG images, it used to take ~38s, it now takes less than 0.5s.
  • For 1000 PNG images, it used to take ~2m39, it now takes 1s.
• "Clear Images" now only clears the images for the active group (meaning the active CameraInit node), instead of clearing all the images for all groups (all CameraInit nodes).
• When using "Clear Images", the "Undo" stack used to be filled with individual intrinsics and viewpoints images. Calling "Clear Images" when 500 images were in the gallery was thus requiring at least 500 calls to "Undo" to be fully reverted. It is now considered as a single action, meaning a single call to "Undo" is enough to recover all the cleared images.

A "Clear All Images" action has been added in the "Advanced" sub-menu to clear the images of all the groups.

Tooltips have been added over both "Clear Images" and "Clear All Images" to make the distinction clear for users.

Features list

• Execute "Clear Images" on the Python side rather than the QML side;
• Make "Clear Images" remove only the images of the current CameraInit node;
• Add a "Clear All Images" action in the "Advanced" sub-menu to remove the images of all the CameraInit nodes.

[mugulmd]

I'm not sure we should be using "Ctrl-D" and "Ctrl-Shift-D" for clearing images since the letter D is usually reserved for duplicating. If that doesn't create any conflict I would rather use the letter X instead, what do you think ?

[fabiencastan]

We don't need to do that very often, so I think we can just remove the shortcut.

[cbentejac]

We don't need to do that very often, so I think we can just remove the shortcut.

I dropped the commit that added the shortcuts.