add path-check, add XPRA_SYSCONF_DIR for xposix, add XDG_DATA_DIRS/icons to search path for pixmaps

[jhgoebbert]

1. The line

LOAD_APPLICATIONS = os.environ.get("XPRA_MENU_LOAD_APPLICATIONS", "%s/share/applications" % sys.prefix).split(":")

https://github.com/Xpra-org/xpra/blob/master/xpra/platform/xposix/menu_helper.py#L35
can return None and therefor needs to be checked for existance before use.

2. Like it is already possible for OSX ( https://github.com/Xpra-org/xpra/blob/master/xpra/platform/darwin/paths.py#L93 )
   it would help if the environment variable XPRA_SYSCONF_DIR can be used to overwrite the system default path.

Example:
We have multiple installations of Xpra on the same multi-user systems of a few hpc-clusters. The user/workflow can choose which of these installations is loaded using lmod, a framework to load software when needed into the current environment.
Xpra is therefore no real system-installations as they are installed to special directories each, and they are no user-installations as they are not installed by the user.
Hence it is needed to avoid that xpra looks at system paths (/etc/xpra) for configuration files.

3. XDG_DATA_DIRS/icons should added to the search paths for icons
   https://specifications.freedesktop.org/icon-theme-spec/latest/ar01s03.html

[totaam]

can return None and therefor needs to be checked for existance before use

No. This cannot return a None value.

Hence it is needed to avoid that xpra looks at system paths

What you want is XPRA_SYSTEM_CONF_DIRS:

xpra/xpra/platform/paths.py

Lines 41 to 45 in 5f5fc0e

	def get_system_conf_dirs():
	return envaslist_or_delegate("XPRA_SYSTEM_CONF_DIRS", do_get_system_conf_dirs)
	def do_get_system_conf_dirs():
	#overriden in all platforms
	return []

[jhgoebbert]

No. This cannot return a None value.

Yes, you are right. But the check is for another reason required as it might not exist:

Python 3.9.6 (default, Nov 16 2021, 12:25:13)
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import os, sys
>>> os.environ.get("XPRA_MENU_LOAD_APPLICATIONS", "%s/share/applications" % sys.prefix).split(":")
['/p/software/juwelsbooster/stages/2022/software/Python/3.9.6-GCCcore-11.2.0/share/applications']
>>>
[goebbert1@jwlogin23 juwels]$ ls /p/software/juwelsbooster/stages/2022/software/Python/3.9.6-GCCcore-11.2.0/share/applications
ls: cannot access '/p/software/juwelsbooster/stages/2022/software/Python/3.9.6-GCCcore-11.2.0/share/applications': No such file or directory

[jhgoebbert]

I will check if XPRA_SYSTEM_CONF_DIRS solves (2) and create a new PR for (1) and (2) then.