Updated dependencies, remove PGM files after encoding

Use the fyne GUI toolkit version 1.4 - better graphics and open/save dialogs.

Updated dependencies, signed the DMG

Version 0.7 has quite some changes, most notable are:

• Added a GUI version, so the program will be a lot easier to use for many people
• Ability to choose your own encoder (including hardware-accelerated ones)
• Decreased binary sizes
• Squashed loads of bugs

Improved error messages, thanks @dga711 for the suggestion

Added support for squeezing videos, see --squeeze. Thanks @naorunaoru for the PR.

Fixes encoding issues with video from the Runcam 5 Orange, thanks to @dangr0

• Fixed non-4:3 aspect rate for input files (#5) - thanks @FieserKiller
• Fixed progress reporting showing > 100%

• Added checks for ffmpeg + h264/h265 support
• Progress bar added
• Better input video specs parsing

Initial release v0.1. Binaries available for Linux (386+amd64), macOS (amd64) and Windows (386 and amd64).