Ensure folder items in sidebar always expandable

[jeremypw]

Fixes #739

Needed to relation to issues with expanding and closing folders in the sidebar as well as accomodating files being added removed and renamed while correctly handling presence/absence of dummy item. The behaviour of the sidebar is now similar to that of Files in relation to empty folders.

• Ensure that empty folders AND folders with no visible children have a dummy entry and therefore expandable
• Simplify handling of created/deleted/renamed files
• Only load all children once; thereafter keep tree in sync with changes
• Prevent stray output stream files being handled or showing.
• Put single use functions inline
• Prevent terminal warning due to attempted updating git status of non-git folders

Test by creating a project with a mixture of subfolders - empty, only non-text files, text files. Add, remove, rename files in each case to check correct behaviour.

[zeebok]

Finally getting to look at this. Code looks good, I just gotta test it.

[zeebok]

When I loaded a folder with a mix of file and folders (the source folder for Code), I noticed non-text files in the build/src directory weren't visible and when I removed what was there, the dummy was not created and thus would not let you expand it. If I restored the deleted files it immediately detected and was expandable.

In my test with fresh new folders, adding and removing things would properly keep the expand functionality. Not sure what is different.

[zeebok]

Just looked at it again after realizing I was running it from the build directory instead of installing it. If I Install it works correctly!

[jeremypw]

It should not matter whether running installed or not (as long there isnt a pre-existing master version running) as no plugins are involved. You can verify which version is running by using meson configure -Ddevelopment=true before building and then checking the window title in multitasking view. Development builds show the branch name in the window title. I am always getting confused about which version of Code is running when developing Code with Code!