Use base color for completion list background.

Fixes unreadable text in completion list with certain color schemes.

BUG: 322849
REVIEW: 113379
Revert "This should finally fix the problem, that the first time the completion popup is shown, the first group is empty."

This reverts commit 51fb673e7f6e02a414505eb5c10b5d4493dac3d9.

This completely breaks code completion with the simple word-based
code completion builtin to Katepart. Probably the issue would arise
for any code completion models which are not grouped at all.

Conflicts:
	part/completion/katecompletionmodel.cpp

CCBUG: 315189
Typo corrections, and making punctuation more consistent in lists and in a table.
Typo corrections, and making punctuation consistent.