BootManager: improved layouting

Addresses #15418 without completely solving it.
At least for font sizes >= 11 points it works mostly. Buttons are
no longer cut off, though the summary page showing all partitions
is taller than necessary for > 3 partitions, esp. for smaller font
sizes.

Views aren't vertically centered at least.

Change-Id: Id6224b83f27510c9ed45e303f0b5dabf47af0574
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7062
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>

* Another work in progress of converting the BootManager to use the layout API;
most pages do now, albeit it's not perfect yet due to issues with BTextView,
and BScrollView that make them very inconvenient to use with the layout API.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@40126 a95241bf-73f2-0310-859d-f6bbb57e9c96

* Renamed bootman to BootManager, and moved it to the applications.
* Renamed its signature to BootManager as well.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@40114 a95241bf-73f2-0310-859d-f6bbb57e9c96

* Another work in progress of converting the BootManager to use the layout API;
most pages do now, albeit it's not perfect yet due to issues with BTextView,
and BScrollView that make them very inconvenient to use with the layout API.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@40126 a95241bf-73f2-0310-859d-f6bbb57e9c96

* Renamed bootman to BootManager, and moved it to the applications.
* Renamed its signature to BootManager as well.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@40114 a95241bf-73f2-0310-859d-f6bbb57e9c96