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>
6ca5573718