Add setting for ui scale using LocalDensity

...op/src/main/kotlin/org/briarproject/briar/desktop/Main.kt

...ject/briar/desktop/contact/add/remote/AddContactDialog.kt

...org/briarproject/briar/desktop/settings/SettingDetails.kt

.../briarproject/briar/desktop/settings/SettingsViewModel.kt

...riarproject/briar/desktop/settings/UnencryptedSettings.kt

...project/briar/desktop/settings/UnencryptedSettingsImpl.kt

...main/kotlin/org/briarproject/briar/desktop/theme/Theme.kt

.../main/kotlin/org/briarproject/briar/desktop/ui/BriarUi.kt

...tlin/org/briarproject/briar/desktop/utils/PreviewUtils.kt

...in/kotlin/org/briarproject/briar/desktop/utils/UiUtils.kt

...esktop/src/main/resources/strings/BriarDesktop.properties

.../org/briarproject/briar/desktop/SetPreviewUtilsDensity.kt