Update ImGuiExt's custom styling to support

[shewitt-au]

Desciption

Update ImGuiExt's custom styling to support the ImGui Push* and Pop* methods for custom colours and styling.

Dependencies

libwolv
pattern_language

Example

You can now do this:

ImGuiExt::PushCustomColor(ImGuiCustomCol_DescButton, IM_COL32(255, 0, 0, 255)); // Added
if (ImGuiExt::BeginSubWindow("hex.builtin.welcome.header.customize"_lang, nullptr, ImVec2(ImGui::GetContentRegionAvail().x - windowPadding, 0), ImGuiChildFlags_AutoResizeX)) {
    if (ImGuiExt::DescriptionButton("hex.builtin.welcome.customize.settings.title"_lang, "hex.builtin.welcome.customize.settings.desc"_lang, ICON_VS_SETTINGS_GEAR, ImVec2(ImGui::GetContentRegionAvail().x, 0)))
        RequestOpenWindow::post("Settings");
}
ImGuiExt::EndSubWindow();
ImGuiExt::PopCustomColor(); // Added

And get this ugliness:

Implementaion Details

The following functions have been added to ImGuiExt. They behave as you would expect:

void PushCustomColor(ImGuiCustomCol idx, ImU32 col);
void PushCustomColor(ImGuiCustomCol idx, const ImVec4& col);
void PopCustomColor(int count = 1);

void PushStyle(ImGuiCustomStyle idx, float val);
void PushStyle(ImGuiCustomStyle idx, const ImVec2 &val);
void PopStyle(int count = 1);

For the custom colour functions the code was lifted and adapted from ImGui's.

The styling functions diverge from ImGui's. ImGui's implementaion is very low level and not easily extended. Instead I used std::variant as the storage in the style stack with some template voodoo. The voodoo was from previous PR I made that was merged, but I moved it into LibWolv. It can be easily extended to support types beyond float and ImVec2.