System information and log separation

TODO list:

• two classes, one shows sysinfo, the other one the log
• two actions, added to the menu etc.
• two separate menu entries visible (not sure why it doesn't work yet...)
• log has sysinfo inside for every session
• there is a new button "Save as text file" in both or just one dialog (better both, since I'm using one widget)
• code is pretty
• commit messages are pretty

Test Plan

1. Go to Help -> Show Krita log for bug reports. See the log with (at least for the latest session) system information.
2. Go to Help -> Show system information for bug reports. See the current system information.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines. needs to be fixed later
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.