Hi all,
May be someone will be interested in this.

I find useless that file is removed from session when I close it from tab, and decided to make 
changes for this.Not sure if the changes are good, this is first working version.

I think that tabs can be more usefull when allows switching only between selected
documents but not between all.

Here are the changes https://github.com/vkochan/geany/commits/close_tab_not_file .
Will be glade to have any response.

Here are some notes:

1) All "close" actions anywhere in IDE now means to close document tab page.

2) Opened tabs are saved into session file for future restoring them after opening the project.

3) To remove document from project it is possible only by using "Remove" menu
item on sidebar panel.

4) Plugins should receive "document-close" event in case when document removed
from project.Not sure if its correct. With these changes should be 2 different events: document-close 
& document-remove.

The feature can be turned on only after defining a macro in cflags before the build:
-DGEANY_CLOSE_TAB_NOT_FILE


I tested it only a little bit, not all cases, so there can be some surprises.