The upgrade concerns the following modules:
- QtCharts
- QtPositioning
- QtLocation
- QtGraphicalEffects
- QtQml

Slots now need to be written as function; the previous syntax is
deprecated.

Setting this variable allows to keep on using "GET" on local files
with XMLHttpRequest.

This commit addresses warnings that were raised by QtCreator's linter:
- IDs declared more than once
- variables declared more than once in the same scope
- type coercions
- variables declared as "var" when their type is known
- unclosed "case" in switch-case

By setting the height on the "onLoaded" event, no height update was
possible unless the component was unloaded (which happens when "active"
gets set to false) first and then reloaded (which happens when "active"
gets set to true). If an attribute was to be hidden (by unchecking the
"advanced attributes" checkbox, for example), its content was effectively
hidden, but as its height was not being updated, a blank block remained
in its place.

Similarly, for attributes that are filled with a lot of content after
being initialized (ChoiceParams, for example), the height was set once
at the attribute's initialization but never updated afterwards.

Manually convert the QPalette object to a QML Palette manually. The
conversion is not straight forward anymore, and not performing it
leads to an incomplete alternate (light) palette.

QML Palette in Qt 5.15 does not support yet disabled elements, the
support will only be enabled with Qt 6.

Qt installation within the dockers is directly handled with command-line
parameters of the installer itself.

…ments

Supporting the alternative palette (triggered with Ctrl+Shift+P) requires
a manual conversion from the QPalette to the QML palette, which does not
directly support disabled elements.

Keeping the previous palette management will support disabled elements,
but cause issues when switching to the alternative palette. Until both
can be fully supported, the alternative palette is disabled.

`libassimpsceneimport.so` is missing from Qt 5.15.2 and must therefore
be added manually. Without it, there will be no OBJ files support in the
3D Viewer.

Add direct download links for `assimpsceneimport.dll` and
`libassimpsceneimport.so` and indications as to where to copy them to
avoid errors related to a missing AssimpSceneParser plugin.