workbench: rearrange initial size of widgets

- Hide console log at first. it'll be visible automatically if needed.
- Set initial width of repo registry to 15em.
- Set vertical stretch of repoview and tasktab to 1:1.
Note: Effective stretch factor is calculated based on initial size.
Since tasktab has more widgets vertically than repoview, tasktab gets
more spaces than repoview.
- Set horizontal stretch of filelist and msg/diff view to 3:7.
- Set vertical stretch of msg and diff to 2:5.
- don't expand revpanel at first.

+------------+-----------------------------------------------------+
| repo reg. | ^ |
| | repoview : |
| | : 1 |
| | v |
| +-----------------------------------------------------+
| | filelist |_____revpanel__________________^_________|
| | | ^ msg : |
| | | v 2 : 1 |
| | +-----------------------------------------+
| | | ^ : |
| | | : diff : |
| | | : 5 : |
| | | v v |
| |<--- 3 --->|<------------------ 7 ------------------>|
|<-- 15em -->| | |
+------------+-----------------------------------------------------+