Consider using a more standard header bar layout

[majjejjam]

Is your feature request related to a problem? Please describe.
GNOME apps with a sidebar and search function usually have the title of the app centered, the search button to the left and the main menu to the right:

Planify has no title and both buttons aligned to the right. This inconsistency is minor, but I thought changing it to follow other apps would improve usability a bit.

Describe the solution you'd like
Add the title of the app to the center of the sidebar's header and move search to the left side.

[golamrabbiazad]

This will be a consistent header design similar to other app's headers. Great catch btw @majjejjam. Nicely describe the feature request with visuals, thanks for that.

I'm working on this now. PR is coming shortly.