Bump qt dependency to qt6

Note that this requires meson 0.63+ because: "before
version 0.63.0 Meson would fail to find Qt 6.1 or later".

Update is to fix a KDE error I was getting when opening
the file selector dialog:
> kf.service.services: KServiceTypeTrader: serviceType
> "ThumbCreator" not found

meson.build

@@ -1,6 +1,6 @@
 project('memoserv', 'cpp',
 	version: '0.1.0',
-	meson_version: '>=0.49.2',
+	meson_version: '>=0.63.0',
 	default_options: ['debug=true', 'cpp_std=c++17', 'b_ndebug=if-release']
 )
 

src/qt/meson.build

@@ -1,10 +1,10 @@
 cpp = meson.get_compiler('cpp')
 fslib_dep = cpp.find_library('stdc++fs', required: false)
 
-qt5 = import('qt5')
-qt5_dep = dependency('qt5', modules: ['Core', 'Gui', 'Widgets'], version: '>=5.14')
+qt6 = import('qt6')
+qt6_dep = dependency('qt6', modules: ['Core', 'Gui', 'Widgets'], version: '>=5.15')
 inc = include_directories('.')
-moc_files = qt5.preprocess(
+moc_files = qt6.preprocess(
 	moc_headers: [
 		'memoserv_win.hpp',
 		'version_info_win.hpp',
@@ -13,7 +13,7 @@ moc_files = qt5.preprocess(
 	#ui_files: 'main_window.ui',
 	moc_extra_arguments: ['-DMAKES_MY_MOC_HEADER_COMPILE'],
 	include_directories: inc,
-	dependencies: qt5_dep
+	dependencies: qt6_dep
 )
 
 app_name = meson.project_name() + '-qt'
@@ -39,7 +39,7 @@ executable(app_name,
 	'animated_pixmap.cpp',
 	include_directories: inc,
 	dependencies: [
-		qt5_dep,
+		qt6_dep,
 		fslib_dep,
 		memcard_dep,
 		vectorwrapper_dep