[Workbench] Open workbench dialog from the sidebar

[WrapEarnPass]

About this:
Having to go to Tools>Menu to open a Workbench seems wasteful when Workbench has a Sidebar available.
Add Sidebar context to allow New and Open for Workbench from Sidebar.
This is less important if #1575 is approved, as that change preloads Workbench in many cases.

[WrapEarnPass]

I noticed that the show/hide block and all the set_sensitive blocks could be skipped a lot of the time, so I added a feature to skip them.

[WrapEarnPass]

Moved this so I could change the Pull request to About this.
I checked out geany@98bcbee3b99d71dfcdd9d3556ace9530ceda818d
and built:
build@debian:~/Projects/geany-plugins-remote/geany-plugins$ $ git log | head
commit 83db05e
No errors raised by make

Making all in src
make[1]: Entering directory '/home/build/Projects/geany-plugins-remote/geany-plugins/workbench/src'
  CC       workbench_la-plugin_main.lo
  CC       workbench_la-workbench.lo
  CC       workbench_la-wb_globals.lo
  CC       workbench_la-wb_project.lo
  CC       workbench_la-wb_monitor.lo
  CC       workbench_la-dialogs.lo
  CC       workbench_la-search_projects.lo
  CC       workbench_la-menu.lo
  CC       workbench_la-popup_menu.lo
  CC       workbench_la-sidebar.lo
  CC       workbench_la-utils.lo
  CC       workbench_la-idle_queue.lo
  CC       workbench_la-tm_control.lo
  CCLD     workbench.la
make[1]: Leaving directory '/home/build/Projects/geany-plugins-remote/geany-plugins/workbench/src'
make[1]: Entering directory '/home/build/Projects/geany-plugins-remote/geany-plugins/workbench'
make[1]: Nothing to be done for 'all-am'.
make[1]: Leaving directory '/home/build/Projects/geany-plugins-remote/geany-plugins/workbench'

No workbench loaded:

New and Open both trigger the same dialog as the Tools>Workbench flow.

Workbench loaded:

Fixes #1571
Contributed under GNU GENERAL PUBLIC LICENSE Version 2