ArmorPaint tries to write in the installation directory and crashes

[alexbruy]

ArmorPaint version: main (8dd3bb0).

OS / GPU model: Linux 6.17.9, NVIDIA GeForce RTX 3060 with NVIDIA driver.

Issue description:
ArmorPaint tries to write files in the installation directory which can be not accessible by the user, e.g. because of the lack of permissions. For example, if ArmorPaint is installed in /opt/armorpaint, attempt to import data (e.g. meshes) from the cloud leads to the attempt to create directory /opt/armorpaint/cloud and subsequent crash.

I believe, it should instead create directories in the user home instead, e.g. $HOME/.share/armorpaint or similar on Linux, %AppData%/armorpaint or similar else on Windows.

While installing ArmorPaint in a writable location (usually user's home directory) is possible, often this is not convenient, e.g. when program should be packaged for OS repositories or when there are multiple users using the same machine.

Steps to reproduce:

1. install ArmorPaint in a location where user don't have write permissions, for example /opt/armorpaint
2. Start ArmorPaint
3. Browse cloud resources and try to download mesh
4. ArmorPaint crashes.