snap to border functionality needs several improvements

[wxl]

The "snap to border" function is not working correctly in several ways, as mentioned in another issue.

Expected Behavior

1. function to turn it off works
2. tooltip provides clarity on function
3. there's a shortcut to customize (or is even exposed)
4. does nothing when there are no borders
5. works correctly on all backgrounds
6. the shortcut always works

Current Behavior

1. can't be turned off
2. tooltip is unhelpful ("Fit to edges only inside selected screen area")
3. shortcut should be customizable
4. works even when no borders are present
5. does not work correctly on certain backgrounds
6. sometimes the shortcut doesn't work (hard to reproduce consistently)

Steps to Reproduce (for bugs)

can't be turned off

1. Click the Settings button
2. Click the Advanced tab
3. Uncheck "Fit to edges inside selected area"
4. Open up pcmanfm-qt
5. Select the Screen Area type
6. Click New
7. Select an area larger than the pcmanfm-qt window
8. Hit space
9. Note the selected area snaps to the border around the pcmanfm-qt window

works even when no borders are present

1. Install a wallpaper with no borders
2. Minimize all windows except screengrab
3. Ensure "Fit to edges inside selected area" is selected in Advanced Settings
4. Select the Screen Area type
5. Click New
6. Select an area
7. Hit space
8. Note that the selected area tries to snap to some non-existent border (often along one axis)

does not work correctly on certain backgrounds

1. Run qterminal
2. Do something like ls -alh so you have some text in the window
3. Run pcmanfm-qt
4. Run screengrab
5. Ensure "Fit to edges inside selected area" is selected in Advanced Settings
6. Select the Screen Area type
7. Click New
8. Select an area larger than the pcmanfm-qt window
9. Hit space
10. Note the selected area snaps to some but not all four borders

System Information

• Distribution & Version: Lubuntu 19.10
• Kernel: 5.0.0
• Qt Version: 5.12.2
• libqtxdg Version: 3.3.1
• lxqt-build-tools Version: 0.6.0
• Package version: 1.101-1ubuntu1