fix(macOS: Electron): Fixed macOS icons for electron engine

[RicardasN]

Description

PNG format is also valid, so added support for that, what is more, ReNative templates were missing icon assignment to the app in macOS dock.

Breaking Changes

• PRs should not introduce breaking changes to existing functionality
• if breaking change cannot be avoided it has to be introduced in 2 phases (release cycles of 0.x.0)
  • 0.x.0 Add new functionality + add DEPRECATED warning to existing fuctionality
  • 0.[x+1].0 Remove deprecated functionality

I have tested my changes on:

ReNative project directly:

• ios simulator
• ios device
• android simulator
• android device
• web browser
• web -e next browser
• tvos simulator
• tvos device
• androidtv simulator
• androidtv device
• androidwear simulator
• androidwear device
• tizen simulator
• tizen device
• tizenmobile simulator
• tizenwatch device
• webos simulator
• webos device
• macos
• windows
• chromecast device

New project:

• ios simulator
• ios device
• android simulator
• android device
• web browser
• web -e next browser
• tvos simulator
• tvos device
• androidtv simulator
• androidtv device
• androidwear simulator
• androidwear device
• tizen simulator
• tizen device
• tizenmobile simulator
• tizenwatch device
• webos simulator
• webos device
• macos
• windows
• chromecast device

Existing Project created with previous version of renative:

• ios simulator
• ios device
• android simulator
• android device
• web browser
• web -e next browser
• tvos simulator
• tvos device
• androidtv simulator
• androidtv device
• androidwear simulator
• androidwear device
• tizen simulator
• tizen device
• tizenmobile simulator
• tizenwatch device
• webos simulator
• webos device
• macos
• windows
• chromecast device