Add some visual separation between toolchain settings

[jonahgraham]

• groups around the global and per-toolchain settings with labels
• whitespace between different toolchain settings
• new logic that shows which projects + build configs are using each displayed toolchain
• added links on the project properties pages back to the global and workspace preferences
• updated the descriptions in the pages

The change in XpackDirectoryNotStrictFieldEditor fixes the implementation to allow doFillIntoGrid to be called multiple times. This is how the other field editors handle it. This is needed now because of how the layout is now being handled with the extra layer of UI hierarchy.

TODO:

• [x] Have this design reviewed - see #574
• [x] Apply the same changes to the other property and preference pages (RISC-V/ARM and Global/Workspace/Project) - 6 classes in total.

Fixes #574

[ilg-ul]

I checked the new look on macOS and it also looks nice. :-)

However something happened in the logic behind the xPack button, since it remains always disabled.

It should be enabled for toolchains installed via xpm, and show the available versions.

[jonahgraham]

However something happened in the logic behind the xPack button, since it remains always disabled.

Oops :-( I have now fixed that and tested it across all the pages that use xpack button. I will push the fix with that in a little while.

[jonahgraham]

@ilg-ul - I am done on this PR. I am happy to fix any bugs and update wording, but unless you feel it is critical I don't think I will do any other refactoring on this.

[ilg-ul]

Merged. Great work, thank you!