platformtheme: Implement DoubleClick/DoubleTap distances

In Qt 5.12.4[1], Qt started checking for the distance between two clicks
or taps when deciding whether a Clicked event should become a
DoubleClicked event. The default distance in Qt 5.12.9 is 5 pixels
horizontally or vertically from the initial click point. The double tap
distance is defined as twice the double click distance[2].

We can use the DPI value of the screen to determine a likely value of
.15 (3/20) of an inch for distance between click events, and .30 of an
inch for touch events. These values, and most of the implementation,
were taken from the Android QPA[3].

Fixes ubports/keyboard-component#159
Fixes ubports/keyboard-component#158

[1] qt/qtdeclarative@56fbc27
[2] https://github.com/qt/qtbase/blob/5.12.9/src/gui/kernel/qplatformtheme.cpp#L548-L563
[3]https://github.com/qt/qtbase/blob/v5.12.9/src/plugins/platforms/android/qandroidplatformtheme.cpp#L469

src/shared/ubuntutheme.h

@@ -16,6 +16,8 @@
 
 #include <QVariant>
 #include <QtThemeSupport/private/qgenericunixthemes_p.h>
+#include <QGuiApplication>
+#include <QScreen>
 
 class UbuntuTheme : public QGenericUnixTheme
 {
@@ -40,6 +42,22 @@ class UbuntuTheme : public QGenericUnixTheme
                 return iconTheme;
             }
         }
+        case QPlatformTheme::MouseDoubleClickDistance: {
+            // Impl mostly yoinked from the QAndroidPlatformTheme
+            QScreen *screen = qGuiApp->primaryScreen();
+            if (screen) {
+                qreal dotsPerInch = screen->physicalDotsPerInch();
+                // Allow 15% of an inch between taps when double clicking
+                return qRound(dotsPerInch * 0.15);
+            } else {
+                return 5;
+            }
+        }
+        case QPlatformTheme::TouchDoubleTapDistance: {
+            bool ok = false;
+            int dist = themeHint(QPlatformTheme::MouseDoubleClickDistance).toInt(&ok) * 2;
+            return QVariant(ok ? dist : 10);
+        }
         default:
             break;
         }