Improve feedback to users when launching the AppImage while the client is already running

[jnweiger]

Pre-submission Checks

• [X] I checked for similar issues, but could not find any. I also checked the closed issues. I could not contribute additional information to any existing issue.
• [X] I will take the time to fill in all the required fields. I know that the bug report may be dismissed otherwise due to lack of information.

Feature description

• I have 4.2.0 running on Linux Mint
• Downloaded the new appimage
• chmod a+x ~/Downloads/ownCloud-5.0.0.12278-x86_64.AppImage
• ~/Downloads/ownCloud-5.0.0.12278-x86_64.AppImage

gui.platform: Core dumps enabled
gui.main: Already running, exiting...

• This is okay, when starting from the command line, the user is informed.
• This is bad, when clicking the appimage in nemo. -> nothing happens.

Expected behaviour:

• a popuip appear, saying ```gui.main: Already running, exiting...`` (the core dumped message should not be shown. It scares users unnecessarily.)

Proposed solution

Expected behaviour:

• a popuip appear, saying ```gui.main: Already running, exiting...`` (the core dumped message should not be shown. It scares users unnecessarily.)

Alternative solutions you considered

No response

Additional context

No response

[fmoc]

This is not an AppImage-specific "problem", it's just the desired user experience.

[TheOneRing]

Well the desired behaviour is that the main instance is raised to the front. Certain Window manager however block non focussed applications from raising windows. Please specify what happens to the main process.

[fmoc]

This is the expected UX and there is -s/--showsettings available (which brings the client to the foreground and is used, e.g., in the desktop entries to allow users to open a running client...