General improvements to stamp annotation

Summary:
Configuration:
- Add push button to select custom stamp image
- Check if loaded image is usable as stamp or throw error
- Keep image proportions in previewer
- Move previewer below the combobox to display larger preview

Annotation tool:
- Keep stamp image proportion in annotation preview (while left mouse button is down)
- Adding the annotation with one-click (without holding the left mouse button and dragging) adds the stamp with original proportions

BUG: 370381
BUG: 383652
CCBUG: 383651
FIXED-IN: 1.9.0

Closes T8074

TODO:
- [ ] Check if filters in file chooser make sense / propose better alternative
- [x] Update doc ( @yurchor will do it after we merge this)

Test Plan:
From stamp annotation configuration dialog:
- Show a warning regarding limitations of the feature's current implementation
- Click push button next to combo box opens a file selector
- Selecting a corrupted image file should throw an error
- Selecting a good image file shows the preview of the image
- Select a horizontal image shows a large clear preview
- Select a vertical image file shows a smaller preview without messing up the visual of the config dialog
- Input a valid icon name in the combobox and the preview of the icon is shown

From page view, select the stamp annotation with horizontal image file (not squared):
- Click and hold. The preview maintains proportions
- Single click. The stamp image in the pdf maintains proportions and has the same size of the click and hold preview.
- Add an annotation of the Okular custom stamps (internal SVG so treated slightly differently) do not create problems

Reviewers: #okular, ngraham

Reviewed By: ngraham

Subscribers: pino, aacid, yurchor, ngraham, okular-devel

Tags: #okular

Maniphest Tasks: T8074

Differential Revision: https://phabricator.kde.org/D22064