Fix various legacy problems, including:
* Don't create default column when viewing an empty project
* Fix layouts for Windows
* Fix (partially) #15509
* Fix (partially) #17705
The sidebar refactoring: it is a clear partial-reloading approach,
brings better user experiences, and it makes "Multiple projects" /
"Project column on issue sidebar" feature easy to be added.
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Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Follow #32460
Now the code could be much clearer than before and easier to maintain. A
lot of legacy code is removed.
Manually tested.
This PR is large enough, that fine tunes could be deferred to the future if
there is no bug found or design problem.
Screenshots:
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