Add noInstall property to qbs module for detecting --no-install option

When working with the `windeployqt` tool, it's necessary to determine if the `--no-install` option has been set from the command line. Currently, I rely on `qbs.installRoot`, but this is more of a hack and not an ideal solution. The check is done by verifying whether the `installRoot` directory exists, which feels like a dirty workaround.

For example:

if (!File.exists(product.qbs.installRoot)) {
    var cmd = new JavaScriptCommand();
    cmd.windeployqtProgram = windeployqtProgram;
    cmd.description = "Cannot invoke '" + windeployqtProgram + "' when '--no-install' option has been set from command line";
    cmd.sourceCode = function () { 
        console.warn("Cannot invoke '" + windeployqtProgram + "' when '--no-install' option has been set from command line (tip: remove your build directory entirely after unchecking '--no-install' option)"); }
} else {
    // Calling windeployqt
}

Proposal:
I suggest adding a noInstall property to the qbs module, which would allow developers to properly detect when the --no-install option has been set without resorting to hacks like checking directory existence.