KRuntimePlatform: add an isMobile function
Should be more reliable than relying on runtimePlatform() which uses a Plasma-specific environment variable.
Shamelessly copied from Kirigami https://invent.kde.org/frameworks/kirigami/-/blob/25446421e672aaf8d3ba7d32be808d4fde75d316/src/platform/settings.cpp#L53-63
Edited by Bart Ribbers