private convenience methods to calculate the text view width and
height with insets and the text rect width and height without
insets respectively.
Also add _ViewRect() and _TextRect() methods for completeness,
but they are not currently used.
Change-Id: I0582bc93a0a3a6820bbb2262a1d726457309ab9f
Reviewed-on: https://review.haiku-os.org/c/haiku/+/7162
Tested-by: Commit checker robot <no-reply+buildbot@haiku-os.org>
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
64a2e73375