Image flag fixes and code consolidation

[Unrealomega]

What's changed:

• Consolidated both plugin files into a single plugin for sanity sake, reducing any development overhead as both plugins did roughly the same thing.
• Added image flags to .tpsheet and .tpset. Now pixel art won't look muddled.
• Removed some excess code and added a bit more error checking.

Known Issues:

• Using Godot 3.2, the below error appears when importing:

* core/os/file_access.cpp:667 - Condition "!f" is true. Continuing.
* Cannot open file 'res://.import/filename.tpset-b0fde670dee72c7687aa825efad333a9.res'. # Only does this for Spritesheets

This does not appear to be related to the plugin, but I can't fully confirm this as I've been unable to nail down what is causing the issue (it doesn't occur when the plugin is installed).

• Using Godot 3.2, every time you alt-tab in and out, it auto-imports. Though the impact is small, it halts development for at least a few seconds and may have some impact on the hardware long-term (reads and write). This issue has existed before this update, at least with my machine.