Add MainWindow/GlobalSearchBoxEnabled (bool) option that controls display of the global search box

Summary:
This allows to disable the box if it crashes for reason unrelated to KEXI (as in bug #390794).

{F5738334}

CCBUG:390794
FIXED-IN:3.1.0

Test Plan:
Set:

```
[MainWindow]
GlobalSearchBoxEnabled=false
```
in kexirc and start KEXI.

Expected: the search box is not created on KEXI startup.

Reviewers: Kexi-Devel-list, piggz

Reviewed By: piggz

Tags: #kexi

Differential Revision: https://phabricator.kde.org/D11000