app: reorganize the Align/Distribute tool options.

This is an alternative to MR !2287 relative to the UX discussions Teams/GIMP/Design/gimp-ux#469.

I believe that many of the proposed designs kinda miss the point by mixing every options. For instance why is the anchor point selection suddenly part of the "Align" section? It also pertains to "Distribute" features.

Why are the selected layers/paths checkbox also going to "Align"? Why is the "Use extents" checkbox on the same level now as "Selected Layers", or even sometimes moved to "Distribute" (whereas it is clearly a sub-options of the "Selected layers")?

This alternative proposal tends to keep globally the same logic as currently, which is 3 sections:

1. The first section is the selection of the objects which will be aligned and/or distributed. This section pertains to both "Align" and "Distribute". You can select layers (and use either the real layer boundary, or the contents boundaries, cropping out any blank space), paths or guides.
2. The second section "Align" contains the "Relative to" option because it pertains only to alignment actions, and of course the few buttons to trigger an alignment.
3. The third section "Distribute" contains the distribution action buttons.

Now the changes I did in this MR so far:

• In the first section, I removed the "Targets" title. I was never really satisfied by this title (which meant "The objects which will be aligned/distributed") but I just can't find a better title. In the end, I'm thinking that no title works out too.
• I added a "Pick guides" checkbox-frame and moved the "Alt-pick target guides" hint message inside the frame (so it only appears when the option is checked), as well as the message indicating how many guides were picked. This way, I believe the guides get on the same level as the layers or paths. It may not have been clear that it's just another type of objects which may be aligned or distributed. Also unchecking the "Pick guides" checkbox un-pick instantly all guides.
• The anchor point widget is now moved inside a popover which appears when clicking an anchor buttons. Near the anchor buttons, there is also a label indicating in plain text what is the chosen anchor: top-left, bottom, center, etc. This should (hopefully) make things clearer and we won't have the issue of this widget disappearing on the side.

Here is a small screencast (note: colors on the guides are weird, it's just the bad screencast compression):

Screencast_From_2025-10-03_19-58-20

@cmyk.student @Reju @rangelovd @aryeom Please tell me what you think.