Improve screenshot path handling (#1815)

* Make --path work correctly with relative paths

Relative paths are taken relative to the working directory of the calling
command, not relative to the daemon's working directory.

* Allow file paths in --path and refactor

* Remove some redundancy

These actions are already performed in the respective functions in
FlameshotDBusAdapter.

* Tweak --path error checker a bit more

* Rework FileNameHandler and update references

The class now has a much simpler interface.

- Screenshot paths are now universally determined by the function
  properScreenshotPath
- Some unreferenced methods have been removed
- The documentation of properScreenshotPath documents the changes well.

* Add crude tests for --path

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Fix failing build on Windows

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Add a test for invalid path

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Make tests clearer

Thanks to @mmahmoudian for his review and contribution.

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Fix bug in properScreenshotPath

Auto-numeration did not work when the screenshot was automatically
saved when copied to clipboard.

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Fall back to default pictures location

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Revert "Remove some redundancy"

This was not redundancy. I had actually introduced a bug with this.

This reverts commit 011ef737564892e494518443e6b80ccf3d286ae1.

* Change default path only on interactive save

Previously, the default save path was changed every time a screenshot
was saved. Now, that only happens when it gets saved from the GUI.

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Change --path help text

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

* Allow other image formats

Signed-off-by: Haris Gušić <harisgusic.dev@gmail.com>

src/core/capturerequest.cpp

@@ -73,7 +73,7 @@ void CaptureRequest::exportCapture(const QPixmap& p)
         if (m_path.isEmpty()) {
             ScreenshotSaver(m_id).saveToFilesystemGUI(p);
         } else {
-            ScreenshotSaver(m_id).saveToFilesystem(p, m_path, "");
+            ScreenshotSaver(m_id).saveToFilesystem(p, m_path);
         }
     }