Cosmetic UI improvements

This project depends on the outcome of QTIFW-1990. If everything is fixed in the linked project, this story becomes obsolete. 

• The two-value radio buttons are not modern (e.g. in the acceptance of the LA). There should be tickbox to select “I agree” and that would guard activation of continue button. QTIFW-2057, QTIFW-2098
• Tooltips should be when there are buttons doing something else than “cancel” or “continue”: QTIFW-2073
• All buttons / text areas should have rounded corners. implemented in qt facelift
• The radio button texts and description texts are not aligned in the default/custom installation selection. QTIFW-2099
• We should notify the user that the installation may take some time. Select components: This component will occupy approximately.... Should we also tell that "You may open the folder and de-select unnecessary components. Those components can be later added using the maintenance tool. QTIFW-2019
• The Essentials description should contain a note that a compatible toolchain is required: "Contains ready-built Qt libraries for MSVC 2020 64-bit toolchain. The toolchain is not included in the installation." QTIFW-2100
• Text in text input fields should be white - green may be difficult to read https://codereview.qt-project.org/c/qtsdk/tqtc-qtsdk-installer-extensions/+/326987
• Sign up should not be a button, but maybe the "Do not have a Qt Account" should be a link directing to the sign up page. Now users click on the sign-up button rather than the Next button. If it remains to be a button, it should be disabled after user starts typing the login credentials. Each person in the test group pressed Sign-up accidentally. QTIFW-2087
• The red colour in the backend error messages could have more brightness to make it more easily readable. Or it could be replaced with the Qt brand yellow. IFW has hard coded red color for error messages (changing with stylesheet not possible), if we change the backend error message color to yellow (or any other) there will be mixed colors used for error messages.
• The backend messages related to the password should be between the password inout box and "Forgot" password. Not feasible? QTIFW-2101
• Ready to install: We should check if the user has sufficient space for the installation and give a warning, if not. We are already doing this, this has been in installer for ages.
• Installation - links to marketed pages in images or below the images. QTIFW-2007
• remove checkerboard pattern-like background from scroll bars on Windows