Import brushes from folder

- A new type of custom brush has been added, brushes from files! Basically there's a "Brushes" folder where Pixelorama can get brushes from, and, unlike the previous brushes, these are for all projects and are not saved in .pxo files. These brushes get loaded on the _ready() method of Main.gd, and are ignored by Godot.
- There are now 2 containers for the two types of custom brushes. The main pixel brush is with the brushes from files.
- Fixed bug where, if you had selected a custom "project" brush and loaded a .pxo file, the brush would still be selected, causing potential problems
- Fixed bug where you could save a project brush that was completely transparent
- Fixed bug where, if you named a file, some shortcuts would be activated.
- export_presets.cfg is now ignored.

export_presets.cfg

@@ -1,32 +0,0 @@
-[preset.0]
-
-name="Windows Desktop"
-platform="Windows Desktop"
-runnable=true
-custom_features=""
-export_filter="all_resources"
-include_filter=""
-exclude_filter=""
-export_path="C:/Users/Overloaded/Dropbox/Orama Interactive/Projects/[Programa Labs]/Pixelorama/Full of bugs/Pixelorama.exe"
-patch_list=PoolStringArray(  )
-script_export_mode=1
-script_encryption_key=""
-
-[preset.0.options]
-
-texture_format/bptc=false
-texture_format/s3tc=true
-texture_format/etc=false
-texture_format/etc2=false
-texture_format/no_bptc_fallbacks=true
-binary_format/64_bits=true
-custom_template/release=""
-custom_template/debug=""
-application/icon=""
-application/file_version="0.3"
-application/product_version="0.3"
-application/company_name="Orama Interactive"
-application/product_name="Pixelorama"
-application/file_description=""
-application/copyright="Orama Interactive 2019"
-application/trademarks=""