[Workbench] Auto open projects

[WrapEarnPass]

Closes #1226
Was tested locally against 2.0 (Debian), but compile checked against master (upstream). There may be a upstream build problem due to that.
Fixes #1561
Contributed under GNU GENERAL PUBLIC LICENSE Version 2

[WrapEarnPass]

I checked out geany@98bcbee3b99d71dfcdd9d3556ace9530ceda818d
and rebuilt:
build@debian:~/Projects/geany-plugins-remote/geany-plugins$ git log | head
commit bc8d253
No errors raised by make

config.status: creating config.h
config.status: config.h is unchanged
config.status: executing depfiles commands
config.status: executing libtool commands
config.status: executing po-directories commands
config.status: creating po/POTFILES
config.status: creating po/Makefile
make[1]: Leaving directory '/home/build/Projects/geany-plugins-remote/geany-plugins'
Making all in src
make[1]: Entering directory '/home/build/Projects/geany-plugins-remote/geany-plugins/workbench/src'
make[1]: Nothing to be done for 'all'.
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'

Created a Workbench, then created a project, then added a project to workbench, then closed and reopened geany.

Both the project and the workbench opened.

This is the only critical in the geany --verbose log, but it appears to be unrelated to the changes.
(geany:109669): Workbench-CRITICAL **: 09:18:59.395: plugin_workbench_on_doc_close: assertion 'doc != NULL && doc->file_name != NULL' failed

[WrapEarnPass]

When I opened this request, I had no idea I would be working on so many issues. I am closing this request to move my changes into a branch so I can keep my fork synced with geany-plugins@master. I opened #1575 to correct that.