QML developers should be able to control when virtual input panel is shown

By default input panel should be shown when editable TextEdit or TextInput gain focus (regardless on how the editor gain the focus), and closed when focus is lost, but developer should also have freedom to disable this behavior and implement a custom mechanism of their own if they want.

Changes:

• remove current input panel mechanism that show panel on mouse release, either in the first or second click depending on the platform
• add boolean property autoShowInputPanelOnFocus that is true by default, if true input panel follows focus
• add openSoftwareInputPanel() and closeSoftwareInputPanel() to manually control sip

Example code:

TextInput {
    id: textInput
    autoShowInputPanelOnFocus: false
    MouseArea {
        anchors.fill: textInput
        onPressed: openSoftwareInputPanel()
    }
}