Add Bengali translation

[maifeeulasad]

I can do this, I guess

[freddii]

Simply make a copy of a file in the folder translations.
For example copy https://github.com/Gr1N/EasyPaint/blob/master/sources/translations/easypaint_fr_FR.ts to a file called easypaint_bn_BN.ts and then replace all the french translations with your bengali ones.

eg:french text for example
tranlated: bengali text for example

then create a pull request of it.

[maifeeulasad]

@freddii thanks, I'll add it along with other commits here